1. Field of the Invention
This invention relates to a crucible used in the production of a silicon single crystal and more particularly to a crucible and a method for pulling a single crystal, which are used in a Czochralski method.
2. Description of the Related Art
Heretofore, the Czochralski method (hereinafter referred to as “CZ method”) is used in the production of a silicon single crystal because of its advantages that a large crystal can be grown and that the speed of crystal growth is high and so on. FIG. 1 is a cross sectional view schematically illustrating a single crystal pulling device used in the CZ method. In this single crystal pulling device 1, a silicon single crystal 70 is grown by contacting a seed crystal 50 attached on a tip end of a pulling axis 40 with a surface of a melt 60 and then pulling the pulling axis upward.
As a crucible used in the single crystal pulling device 1 is generally used a crucible 10 for pulling a single crystal with a double structure comprising a silica crucible 20 and a graphite crucible 30 covering the outside of the silica crucible 20 as shown in FIG. 1.
The upper end of the crucible 10 is usually designed so that the side wall of the silica crucible is made higher than that of the graphite crucible. Therefore, when the crucible is exposed to a higher temperature during the pulling of a silicon single crystal, there are problems that the upper portion of the silica crucible is fallen down inward in a radial direction of the crucible and that buckling occurs due to the own weight of the silica crucible. In such cases, it is obliged to stop the pulling of the silicon single crystal.
FIGS. 4 and 5 are sectional side elevations illustrating deformations of the silica crucible generated during the pulling of the silicon single crystal, respectively. FIG. 4 illustrates a state that an upper end of a body portion in the silica crucible 20 is fallen down inward in a radial direction of the crucible (hereinafter referred to as “inward falling of the body portion in the silica crucible”). As the inward falling progresses, the body portion of the silica crucible 20 finally falls down into the melt 60, which adversely affects the pulling of the silicon single crystal. On the other hand, FIG. 5 illustrates a buckling state generated due to the own weight of the silica crucible, which also affects the pulling of the silicon single crystal adversely.
Therefore, there is disclosed a technique for preventing the above buckling and inward falling by rationalizing the shape of the silica crucible. For example, as disclosed in JP-A-H06-32692 and JP-A-H09-249494, there is a crucible for pulling a silicon single crystal, which comprises a silica crucible having a taperingly form of expanding a diameter of an upper portion upwardly and a graphite crucible.
However, the crucible for pulling a silicon single crystal as disclosed in JP-A-H06-32692 and JP-A-H09-249494 has a certain effect on the control of the buckling generation because the going down of the silica crucible can be suppressed based on the contact of the upper end portion of the graphite crucible with the upper portion of the silica crucible, whereas the control on the inward falling of the body portion in the silica crucible is not sufficient because there is a risk of causing the falling as shown in FIG. 6.